How do I use uploaded files while previewing in Construct 3?

I have uploaded png and css files in the Files section of the Project bar but I can't seem to locate where these files appear while previewing the project.

When the game is exported, the files are saved in the root directory and can be accessed using "filename.png" (for example, background-image: url("filename.png") in a css file.

However, in the web browser while previewing the game, "filename.png" does not work, nor does "files/filename.png" or "Files/filename.png".

For loading CSS files from the files, I am running the script:

link = document.createElement("link");
link.type = "text/css";
link.rel = "stylesheet";
link.href = "style.css";
document.head.appendChild(link);